The present invention relates to an optical beam scanning device that irradiates image light modulated by a signal based on image data to the surface of an image bearing member, used in an image forming apparatus such as a printer, facsimile, copy machine, or the like that performs electrophotographic image formation, and an image forming apparatus provided with this optical beam scanning device.
As an optical beam scanning device of an image forming apparatus, laser beam systems are widely used that make laser light, which has a fast modulation speed and can increase scanning speed, into image light.
In recent years, conversion of image forming apparatuses to color has been advancing, and various optical beam apparatuses for color image formation have been developed. Conventional optical beam scanning devices for color image formation have been provided one for each component color, and an optical beam scanning device must be disposed in the image forming apparatus for each color, and along with increased size of the image forming apparatus, expensive optical parts are redundantly provided, leading to increased cost.
Accordingly, as disclosed in JSP 2001-235699A and JSP 2001-281587A, technology has been proposed wherein multiple laser light sources for each component color are provided in a single optical beam scanning device. In this optical beam scanning device, mirrors for each component color are housed along with laser light sources for each component color, a single polygon mirror, and a single fθ lens in a single housing, and after the light beam emitted from each laser light source is deflected at a constant angular velocity by the single polygon mirror and deflected at a constant velocity by the single fθ lens, it is separated and dispersed toward the image bearing member for each component color via the mirror provided for each component color.
However, in the optical beam scanning devices disclosed in JSP 2001-235699A and JP 2001-281587A, the mirrors for each component color, the laser light sources, the polygon mirror, and the fθ lens are housed in a single housing, therefore leading to the problem of increased size of the apparatus.
In order to prevent the deterioration of color image quality due to the occurrence of color shift, it is necessary to strictly maintain the relative position of optical parts such as the laser light sources, polygon mirror, fθ lens, and mirrors, and their position relative to the image bearing members, and the influence of environmental conditions on the optical beam scanning device, particularly changes in temperature, must be made small.
Thus, using expensive and heavy resin material with a small coefficient of linear expansion such as BMC (Bulk Molding Compound) for the material of the housing, which is a large member that supports all of the optical parts, leads to a jump in the cost of parts and increased weight.
This sort of problem occurs not only in an optical beam scanning device used in a color image forming apparatus that forms a color image, but also in an optical beam scanning device used in an image forming apparatus that forms a monochrome image.
It is an object of this invention to provide an optical beam scanning device that can realize lower cost and weight by reducing the amount of expensive and heavy materials used as much as possible, and to provide an image forming apparatus provided with that optical beam scanning device.